Recently, mobile radio telecommunication systems have widely spread. Such mobile radio telecommunication systems operate for example according to a commonly agreed standard, like for example the GSM standard. According to GSM standard, data transmission is performed according to a method of time divisional multiple access (TDMA). The TDMA transmission principle specifies that data are transmitted from a transmitter to a receiver and vice versa only during respectively specified time slots of frames.
Data transmission in such telecommunication systems substantially relies on digital data transmission. However, between a mobile radio transceiver device as a subscriber terminal (hereinafter: mobile station MS) and a stationary radio transceiver device as a serving radio network element (hereinafter: base station BS) data have necessarily to be transmitted as analogue data via the air interface Um (representing the transmission channel of interest).
Consequently, data transmitted by the mobile station MS are received via a base station antenna means of a reception device of the base station BS as analogue data. In the course of the further processing of the thus received data by the reception device, the analogue data are analog to digital converted, i.e. passed through an A/D converter means. At the next stage of the processing, the obtained digital data are supplied to an equalizer means for being equalized. The thus obtained output data are then supplied to a channel codec means for coding/decoding the data. At the reception side, decoding is performed to separate received speech data from associated signaling data.
Particular attention in the course of this processing has to be paid to the equalizing of the received data, since the equalizing is required to reconstruct, at the reception side, the transmitted signal from a mixture of received signals.
For example, assuming a situation in a radio telecommunication network with a base station BS and only a single mobile station MS present in the radio coverage area of the base station. Then, a signal s transmitted from the mobile station MS may reach the base station BS directly via line of sight at a time s(t). However, the same signal s may be deflected by, e.g., a building, a mountain or the like present in the environment. Thus, the same signal may reach the base station BS at a later point of time s(t+T), and is thus superposed to the signal s(t). Due to the delay T, both received signals are no longer in phase with each other. Moreover, the delayed signal s(t+T) may even be more attenuated than the signal s(t) due to the longer transmission path. Thus, the signal received by the base station BS and originating from the mobile station MS is distorted. Now, assuming that another mobile station MS′ is additionally present, then signals s′(t′), s′(t′+T′) are additionally received by the base station BS, which may lead to interference between the respective transmitted data symbols (intersymbol interference).
Therefore, an equalizer means has to reconstruct (detect) the initially transmitted signal s(t) and/or s′(t′) from the received mixture of signals s(t), s(t+T), s′(t′), s′(t′+T′).
The thus reconstructed (or detected) signal is required to be as similar to the originally transmitted signal as possible. This reconstruction is therefore a main concern when designing equalizers, e.g. for use in a reception device (receiver) of a base station BS.
As regards the adopted channel model in such a case, each of the above mentioned delayed signals forms a term of a respective FIR model of the channel impulse response function and represents a so-called tap. Based on the values of such taps, poles and zeroes of the transmission function can be calculated.
On a receiver side, it is necessary to have a knowledge of the quality of the received signal. This quality of the received signal is represented by the noise energy (variance) of the received signal. The knowledge of signal quality as represented by the noise energy is then used in a respective receiver (e.g. in connection with equalization).
In mobile communication systems, the noise energy of the received signal has to be estimated from the stream of received data.
According to previous solutions, the noise energy is calculated from the difference between the received signal on one hand and the result of a convolution processing of a training sequence (known to the receiver and also transmitted via the transmission channel and therefore contained in the received signal) and an estimated channel impulse response. An example thereof is described in document EP-B1-0 428 199.
However, performing a convolution processing requires a powerful digital signal processor device at the receiver side with a large data memory for intermediate storage of the convolution results. Moreover, due to the complexity of a convolution processing, such a processing is also rather time consuming.
Additionally, in receivers which are particularly adapted to receive and reconstruct transmitted speech data, noise energy is estimated based on the Viterbi algorithm. An example thereof is described in applicants former document WO 97/08841.
However, also an implementation of the Viterbi algorithm requires a powerful digital signal processor device at the receiver side with a large data memory for intermediate storage of the convolution results.